The Committee Consultative International Telephone and Telegraphique (CCITT) has drafted standards for Integrated Services Digital Networks (ISDN); the so-called "I-Series Recommendations". The I.400 Series Recommendations define the User-Network Interface; in particular, the I.430 Recommendations specify so-called INFO 0, INFO 1, INFO 2, INFO 3 and INFO 4 signals at the "S" and "T" reference points. Familiarity with these Recommendations is helpful to an understanding of the instant invention and they are incorporated by reference herein.
The INFO 1 signal is defined as a signal from terminal equipment (TE) to network termination (NT) consisting of a continuous pattern of a positive-going pulse ("HIGH mark") followed by a negative-going pulse ("LOW mark"), followed by a series of six bit-time periods of no pulses (six "spaces"); the nominal speed of rotation being 192 kilobits per second (kbs).
Upon reception of the INFO 1 signal, activation of the NT equipment should begin as quickly as possible to avoid loss of data and/or to minimize transmission times. On the other hand, false activation of the NT equipment should be avoided. Presently, signals on the line upon which data is transmitted and sampled at the 192 kbs rate. Reception of the high mark and low mark during any consecutive eight bit time periods causes activation of the NT. Such a technique may cause false activation of the line because a noise signal may contain a HIGH mark and a LOW mark during eight consecutive bit time periods.
Another problem with this technique is that the clock used by the TE for transmission is asynchronous with respect to the clock used by the NT, prior to, and during activation. Furthermore, several TE's could attempt simultaneous activation requests, resulting in several asynchronous INFO 1 signals becoming superimposed on the line.